1. Field of the Invention
The present invention is in the field of medicinal chemistry and relates to compounds that have a high affinity for the glycine binding site, lack PCP side effects, and cross the blood brain barrier at high levels. In particular, the present invention relates to novel alkyl, azido, alkoxy, fluoro-substituted, and fused 1,4-dihydroquinoxaline-2,3-diones and their use to treat or prevent neuronal degeneration associated with ischemia, pathophysiologic conditions associated with neuronal degeneration, convulsions, anxiety, chronic pain, and to induce anesthesia.
2. Description of the Related Art
Glutamate is thought to be the major excitatory neurotransmitter in the brain. There are three major subtypes of glutamate receptors in the CNS. These are commonly referred to as kainate, AMPA, and N-methyl-D-aspartate (NMDA) receptors (Watkins and Olverman, Trends in Neurosci. 7:265-272 (1987)). NMDA receptors are found in the membranes of virtually every neuron in the brain. NMDA receptors are ligand-gated cation channels that allow Na.sup.+, K.sup.+, and Ca.sup.++ to permeate when they are activated by glutamate or aspartate (non-selective, endogenous agonists) or by NMDA (a selective, synthetic agonist) (Wong and Kemp, Ann. Rev. Pharmacol. Toxicol. 31:401-425 (1991)).
Glutamate alone cannot activate the NMDA receptor. In order to become activated by glutamate, the NMDA receptor channel must first bind glycine at a specific, high affinity, glycine binding site that is separate from the glutamate/NMDA binding site on the receptor protein (Johnson and Ascher, Nature 325:329-331 (1987)). Glycine is therefore an obligatory coagonist at the NMDA receptor/channel complex (Kemp, J. A., et al., Proc. Natl. Acad. Sci. USA 85:6547-6550 (1988)).
In addition to the binding sites for glutamate/NMDA and glycine, the NMDA receptor carries a number of other functionally important binding sites. These include binding sites for Mg.sup.++, Zn.sup.++, polyamines, arachidonic acid, and phencyclidine (PCP) (Reynolds and Miller, Adv. in Pharmacol. 21:101-126 (1990); Miller, B., et al., Nature 355:722-725 (1992)). The PCP binding site--now commonly referred to as the PCP receptor--is located inside the pore of the ionophore of the NMDA receptor/channel complex (Wong, E. H. F., et al., Proc. Natl. Acad. Sci. USA 83:7104-7108 (1986); Huettner and Bean, Proc. Natl. Acad. Sci. USA 85:1307-1311 (1988); MacDonald, J. F., et al., Neurophysiol. 58:251-266 (1987)). In order for PCP to gain access to the PCP receptor, the channel must first be opened by glutamate and glycine. In the absence of glutamate and glycine, PCP cannot bind to the PCP receptor although some studies have suggested that a small amount of PCP binding can occur even in the absence of glutamate and glycine (Sircar and Zukin, Brain Res. 556:280-284 (1991)). Once PCP binds to the PCP receptor, it blocks ion flux through the open channel. Therefore, PCP is an open channel blocker and a non-competitive glutamate antagonist at the NMDA receptor/channel complex.
One of the most potent and selective drugs that bind to the PCP receptor is the anticonvulsant drug MK801. This drug has a K.sub.d of approximately 3 nM at the PCP receptor (Wong, E. H. F., et al., Proc. Natl. Acad. Sci. USA 83:7104-7108 (1986)).
Both PCP and MK801 as well as other PCP receptor ligands, e.g., dextromethorphan, ketamine, and N,N'-disubstituted guanidines, have neuroprotective efficacy both in vitro and in vivo (Gill, R., et al., J. Neurosci. 7:3343-3349 (1987); Keana, J. F. W., et al., Proc. Natl. Acad. Sci. USA 86:5631-5635 (1989); Steinberg, G. K., et al. , Neuroscience Lett. 89:193-197 (1988); Church, J., et al., In: Sigma and Phencyclidine-Like Compounds as Molecular Probes in Biology, Domino and Kamenka, eds., Ann Arbor: NPP Books, pp. 747-756 (1988)). The well-characterized neuroprotective efficacy of these drugs is largely due to their capacity to block excessive Ca.sup.++ influx into neurons through NMDA receptor channels, which become over activated by excessive glutamate release in conditions of brain ischemia (e.g. in stroke, cardiac arrest ischemia etc.) (Collins, R. C., Metabol. Br. Dis. 1:231-240 (1986); Collins, R. C., et al., Annals Int. Med. 110:992-1000 (1989)).
However, the therapeutic potential of these PCP receptor drugs as ischemia rescue agents in stroke has been severely hampered by the fact that these drugs have strong PCP-like behavioral side effects (psychotomimetic behavioral effects) which appear to be due to the interaction of these drugs with the PCP receptor (Tricklebank, M. D., et al., Eur. J. Pharmacol. 167:127-135 (1989); Koek, W., et al., J. Pharmacol. Exp. Ther. 245:969 (1989); Willets and Balster, Neuropharmacology 27:1249 (1988)). These PCP-like behavioral side effects appear to have caused the withdrawal of MK801 from clinical development as an ischemia rescue agent. Furthermore, these PCP receptor ligands appear to have considerable abuse potential as demonstrated by the abuse liability of PCP itself.
The PCP-like behavioral effects of the PCP receptor ligands can be demonstrated in animal models: PCP and related PCP receptor ligands cause a behavioral excitation (hyperlocomotion) in rodents (Tricklebank, M. D., et al., Eur. J. Pharmacol. 167:127-135 (1989)) and a characteristic catalepsy in pigeons (Koek, W., et al., J. Pharmacol. Exp. Ther. 245:969 (1989); Willets and Balster, Neuropharmacology 27:1249 (1988)); in drug discrimination paradigms, there is a strong correlation between the PCP receptor affinity of these drugs and their potency to induce a PCP-appropriate response behavior (Zukin, S. R., et al., Brain Res. 294:174 (1984); Brady, K. T., et al., Science 215:178 (1982); Tricklebank, M. D., et al., Eur. J. Pharmacol. 141:497 (1987)).
Drugs acting as competitive antagonists at the glutamate binding site of the NMDA receptor, such as, CGS 19755 and LY274614, also have neuroprotective efficacy because these drugs--like the PCP receptor ligands--can prevent excessive Ca.sup.++ flux through NMDA receptor/channels in ischemia (Boast, C. A., et al., Brain Res. 442:345-348 (1988); Schoepp, D. D., et al., J. Neural. Trans. 85:131-143 (1991)). However, competitive NMDA receptor antagonists also have PCP-like behavioral side-effects in animal models (behavioral excitation, activity in PCP drug discrimination tests) although not as potently as MK801 and PCP (Tricklebank, M. D., et al., Eur. J. Pharmacol. 167:127-135 (1989)).
An alternate way of inhibiting NMDA receptor channel activation is by using antagonists at the glycine binding site of the NMDA receptor. Since glycine must bind to the glycine site in order for glutamate to effect channel opening (Johnson and Ascher, Nature 325:329-331 (1987); Kemp, J. A., et al., Proc. Natl. Acad. Sci. USA 85:6547-6550 (1988)), a glycine antagonist can completely prevent ion flux through the NMDA receptor channel--even in the presence of a large amount of glutamate.
Recent in vivo microdialysis studies have demonstrated that, in the rat focal ischemia model, there is a large increase in glutamate release in the ischemic brain region with no significant increase in glycine release (Globus, M. Y. T., et al., J. Neurochem. 57:470-478 (1991)). Thus, theoretically, glycine antagonists should be very powerful neuroprotective agents because they can prevent the opening of NMDA channels by glutamate non-competitively and, therefore, unlike competitive NMDA antagonists, do not have to overcome the large concentrations of endogenous glutamate that are released in the ischemic brain region.
Furthermore, because glycine antagonists act at neither the glutamate/NMDA nor the PCP binding sites to prevent NMDA channel opening, these drugs might not cause the PCP-like behavioral side effect seen with both PCP receptor ligands and competitive NMDA receptor antagonists (Tricklebank, M. D., et al., Eur. J. Pharmacol. 167:127-135 (1989); Koek, W., et al., J. Pharmacol. Exp. Ther. 245:969 (1989); Willets and Balster, Neuropharmacology 27:1249 (1988); Tricklebank, M. D., et al., Eur. J. Pharmacol. 167:127-135 (1989); Zukin, S. R., et al., Brain Res. 294:174 (1984); Brady, K. T., et al., Science 215:178 (1982); Tricklebank, M. D., et al., Eur. J. Pharmacol. 141:497 (1987)). That glycine antagonists may indeed be devoid of PCP-like behavioral side effects has been suggested by recent studies in which available glycine antagonists were injected directly into the brains of rodents without resulting in PCP-like behaviors (Tricklebank, M. D., et al., Eur. J. Pharmacol. 167:127-135 (1989)).
However, there have been two major problems that have prevented the development of glycine antagonists as clinically useful neuroprotective agents:
A. Most available glycine antagonists with relatively high receptor binding affinity in vitro such as 7-Cl-kynurenic acid (Kemp, J. A., et al., Proc. Natl. Acad. Sci. USA 85:6547-6550 (1988)), 5,7-dichlorokynurenic acid (McNamara, D., et al., Neuroscience Lett. 120:17-20 (1990)) and indole-2-carboxylic acid (Gray, N. M., et al., J. Med. Chem. 34:1283-1292 (1991)) cannot penetrate the blood/brain barrier and therefore have no utility as therapeutic agents; PA1 B. The only available glycine antagonist that sufficiently penetrates the blood/brain barrier--the drug HA-966 (Fletcher and Lodge, Eur. J. Pharmacol. 151:161-162 (1988))--is a partial agonist with only micromolar affinity for the glycine binding site. A neuroprotective efficacy for HA-966 in vivo has, therefore, not been demonstrated, nor has it been demonstrated for the other available glycine antagonists because they lack bioavailability in vivo. PA1 R.sup.6 and R.sup.7 independently are NO.sub.2, halogen, CN, CF.sub.3, or OR', wherein R' is C.sub.1-4 -alkyl, and R.sup.5 and R.sup.8 are each hydrogen; or PA1 R.sup.5 and R.sup.6 together form a further fused aromatic ring, which may be substituted with halogen, NO.sub.2, CN, CF.sub.3, or OR', wherein R' is C.sub.1-4 -alkyl; or PA1 R.sup.7 and R.sup.8 together form a further fused aromatic ring, which may be substituted with halogen, NO.sub.2, CN, CF.sub.3, or OR', wherein R' is C.sub.1-4 -alkyl, and R.sup.5 and R.sup.6 independently are hydrogen, halogen, CN, CF.sub.3, NO.sub.2, or OR', wherein R' is C.sub.1-4 -alkyl. These compounds are reportedly useful for the treatment of indications caused by hyperactivity of the excitatory neurotransmitters, particularly the quisqualate receptors, and as neuroleptics. PA1 lack the PCP-like behavioral side effects common to the PCP-like NMDA channel blockers, such as, MK801, or to the competitive NMDA receptor antagonists, such as, CGS 19755; PA1 show potent anti-ischemic efficacy because of the non-competitive nature of their glutamate antagonism at the NMDA receptor; PA1 cross the blood-brain barrier at levels sufficient for efficacy; PA1 have utility as novel anticonvulsants with fewer side-effects than the PCP-like NMDA channel blockers or the competitive NMDA antagonists; PA1 help in defining the functional significance of the glycine binding site of the NMDA receptor in vivo. PA1 R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are independently selected from the group consisting of hydrogen, hydroxy, acyloxy, aralkoxy, amino, alkanoylamino, halo, haloalkyl, nitro, alkyl, alkoxy, carboxy, alkanoyl, thioalkyl, alkoxyalkyl, aryloxyalkyl, arylalkyl, alkenyl, alkynyl, arylalkenyl, arylalkynyl, cyano, cyanomethyl, dicyanomethyl, cyanoamino, dicyanoamino, or azido; PA1 R.sup.2 is fluoro, chloro, alkyl, alkoxy, or azido; PA1 R.sup.3 is fluoro or chloro; and PA1 R.sup.4 is hydrogen or fluoro; PA1 R.sup.1 is nitro, cyano, CF.sub.3, carboxy, or alkanoyl; PA1 R.sup.2 is alkoxy, aralkoxy, thioalkyl, carboxy, alkanoyl, hydroxy, mercaptoalkyl, azido, or NR.sup.5 R.sup.6, wherein R.sup.5 and R.sup.6 are independently hydrogen, alkyl, or aryl groups; PA1 R.sup.3 is halo, haloalkyl, nitro, alkyl, alkoxy, azido, or cyano; and PA1 R.sup.4 is hydrogen. PA1 R.sup.2 is haloalkyl, halo, cyano, alkyl, or alkoxy; PA1 R.sup.3 is haloalkyl, halo, cyano, alkyl, or alkoxy; and PA1 R.sup.4 is hydrogen; PA1 R.sup.2 is haloalkyl, halo, cyano, alkyl, or alkoxy; PA1 R.sup.3 is haloalkyl, halo, cyano, alkyl, or alkoxy; and PA1 R.sup.4 is hydrogen; PA1 R.sup.2 is alkoxy, aralkoxy, hydroxy, mercaptoalkyl, azido, or NR.sup.5 R.sup.6, wherein R.sup.5 and R.sup.6 are independently hydrogen, alkyl, or aryl groups; PA1 R.sup.3 is halo, haloalkyl, nitro, alkyl, alkoxy, azido, or cyano; and PA1 R.sup.4 is hydrogen; PA1 R.sup.2 is fluoro; PA1 R.sup.3 is halo, haloalkyl, nitro, alkyl, alkoxy, azido, or cyano; and PA1 R.sup.4 is hydrogen; PA1 R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are independently selected from the group consisting of hydrogen, hydroxy, acyloxy, aralkoxy, amino, alkanoylamino, halo, haloalkyl, nitro, alkyl, alkoxy, carboxy, alkanoyl, thioalkyl, alkoxyalkyl, aryloxyalkyl, arylalkyl, alkenyl, alkynyl, arylalkenyl, arylalkynyl, cyano, cyanomethyl, dicyanomethyl, cyanoamino, dicyanoamino, or azido;
However, one recent success in identifying orally active glycine receptor antagonists was reported by Kulagowski et al., J. Med. Chem. 37:1402-1405 (1994), who disclose that 3-substituted 4-hydroxyquinoline-2(1H)-ones are selective antagonists possessing potent potent in vivo activity.
There have been a number of reports in the literature of substituted 1,4-dihydroquinoxaline-2,3-diones that are useful for treating pathophysiologic conditions mediated by the non-NMDA, NMDA, and glycine receptors. For example, U.S. Pat. No. 4,975,430 discloses 1,4-dihydroquinoxaline-2,3-dione compounds of the formula: ##STR1## wherein each X is independently nitro or cyano and wherein each Y is independently H, lower alkyl, lower alkoxy, or CF.sub.3. These compounds are reportedly useful for the treatment of neuronal conditions associated with stimulation of the NMDA receptor.
U.S. Pat. No. 3,962,440 discloses 1,4-dihydroquinoxaline-2,3-dione compounds having the formula: ##STR2## wherein, R.sup.1 can be hydrogen or methyl, R.sub.n can be lower alkyl, lower alkoxy, lower alkylthio, cyclopropyl, nitro, cyano, halogen, fluoroalkyl of C.sub.1 -C.sub.2 (trifluoromethyl) amino, or substituted amino, and n can be 0, 1, or 2. These compounds are reportedly useful as hypnotic agents.
U.S. Pat. No. 4,812,458 discloses 1,4-dihydroquinoxaline-2,3-dione compounds having the formula: ##STR3## wherein R.sup.1 is halogen, cyano, trifluoromethyl, ethynyl, or N.sub.3, and R.sup.2 is SO.sub.2 C.sub.1-3 -alkyl, trifluoromethyl, nitro, ethynyl, or cyano. These compounds are reportedly useful for treatment of indications caused by hyperactivity of the excitatory neurotransmitters, particularly the quisqualate receptors, and as neuroleptics.
U.S. Pat. No. 4,659,713 discloses 1,4-dihydroquinoxaline-2,3-dione compounds having the formula: ##STR4## wherein X represents hydrogen, chloro, bromo, fluoro, iodo, trichloromethyl, dichlorofluoromethyl, difluoromethyl, or trifluoromethyl, and n represents 1 or 2. These compounds are reportedly useful for the control of coccidiosis in animals.
U.S. Pat. No. 4,948,794 discloses 1,4-dihydroquinoxaline-2,3-dione compounds having the formula: ##STR5## wherein R.sup.1 is C.sub.1-12 -alkyl, which may, optionally, be substituted by hydroxy, formyl, carboxy, carboxylic esters, amides, or amines, C.sub.3-8 -cycloalkyl, aryl, aralkyl; and wherein R.sup.6 is, hydrogen, halogen, CN, CF.sub.3, NO.sub.2, or OR', wherein R' is C.sub.1-4 -alkyl, and R.sup.5, R.sup.7, and R.sup.8 are hydrogen, provided R.sup.6 is not CF.sub.3, OCH.sub.3, NO.sub.2, Cl, or Br when R.sup.1 is CH.sub.3 ; or
Yoneda and Ogita, Biochem. Biophys. Res. Commun. 164:841-849 (1989), disclose that the following 1,4-dihydroquinoxaline-1,2-dione competitively displaced the strychnine-insensitive binding of [.sup.3 H]glycine, without affecting the other binding sites on the NMDA receptor complex: ##STR6##
According to the authors, the structure-activity relationships among quinoxalines clearly indicates that both chloride groups of the positions 6 and 7 in the benzene ring are crucial for the antagonist potency against the Gly sites. Removal of one chloride from the molecule results in a 10-fold reduction in the affinity for Gly sites.
Kleckner and Dingledine, Mol. Pharm. 36:430-436 (1989), disclose that 6,7-dinitro-1,4-dihydroquinoxaline-2,3-dione and 6-cyano-7-nitro-1,4-dihydroquinoxaline-2,3-dione are more potent antagonists of kainate than glycine, but substitution of Cl at the 6-position and especially at the 6- and 7-positions increases potency at the glycine site. In addition, the authors suggest that antagonists of the glycine site might be effective against NMDA receptor-mediated neuropathologies.
Rao, T. S. et al., Neuropharmacology 29:1031-1035 (1990), disclose that 6,7-dinitro-1,4-dihydroquinoxaline-2,3-dione and 7-cyano-6-nitro-1,4-dihydroquinoxaline-2,3-dione antagonize responses mediated by NMDA-associated glycine recognition sites in vivo.
Pellegrini-Giampietro, D. E. et al., Br. J. Pharmacol. 98:1281-1286 (1989), disclose that 6-cyano-7-nitro-1,4-dihydroquinoxaline-2,3-dione and 6,7-dinitro-1,4-dihydroquinoxaline-2,3-dione can antagonize the responses to L-glutamate by interacting with the glycine recognition sites of the NMDA receptor ion channel complex.
Ogita and Yoneda, J. Neurochem. 54:699-702 (1990), disclose that 6,7-dichloro-1,4-dihydroquinoxaline-2,3-dione is a competitive antagonist specific to the strychnine-insensitive [.sup.3 H] glycine binding sites on the NMDA receptor complex. According to the authors, the two chloride radicals at the 6- and 7-positions in the benzene ring of the quinoxaline are crucial for the antagonistic potency against the glycine binding sites.
Kessler, M. et al., Brain Res. 489:377-382 (1989), disclose that 6,7-dinitro-1,4-dihydroquinoxaline-2,3-dione and 6-cyano-7-nitro-1,4-dihydroquinoxaline-2,3-dione inhibit [.sup.3 H] glycine binding to the strychnine-insensitive glycine binding sites associated with NMDA receptors.
European Patent Application Publication No. 0 377 112, published Jul. 11, 1990, discloses 1,4-dihydroquinoxaline-2,3-dione compounds having the formula: ##STR7## wherein, inter alia, R.sup.1 can be hydroxy, alkoxy, aryloxy, aralkyloxy, cycloalkylalkoxy, cycloalkoxy, or alkanoyloxy; and R.sup.5, R.sup.6, R.sup.7 and R.sup.8 can be independently hydrogen, nitro, halogen, cyano, trifluoromethyl, SO.sub.2 NR'R', SO.sub.2 R' or OR', wherein R' is hydrogen or C.sub.1-4 alkyl. These compounds are reportedly useful for the treatment of indications caused by hyperactivity of the excitatory neurotransmitters, particularly the quisqualate receptors, and as neuroleptics.
Lester, R. A. et al., Mol. Pharm. 35:565-570 (1989), disclose that 6-cyano-7-nitro-1,4-dihydroquinoxaline-2,3-dione antagonizes NMDA receptor-mediated responses by a competitive interaction of the glycine binding site.
Patel, J. et al., J. Neurochem. 55:114-121 (1990), disclose that the neuroprotective activity of 6,7-dinitro-1,4-dihydroquinoxaline-2,3-dione is due to antagonism of the coagonist activity of glycine at the NMDA receptor-channel complex.
Horner, L. et al., Chem. Abstracts 48:2692 (1953) disclose 6,8-dinitro-1,4-dihydroquinoxaline-2,3-dione.
Cheeseman, G. W. H., J. Chem. Soc.: 1170-1176 (1962), discloses 6,7-dibromo-2,3-dihydroxyquinoxaline (also known as 6,7-dibromo-1,4-dihydroquinoxaline-2,3-dione).
Honore, T. et al., Science 241:701-703 (1988), disclose that 6,7-dinitro-1,4-dihydroquinoxaline-2,3-dione and 7-cyano-6-nitro-1,4-dihydroquinoxaline-2,3-dione are potent non-NMDA glutamate receptor antagonists.
Sheardown, M. J. et al., Eur. J. Pharmacol. 174:197-204 (1989), disclose that 5,7-dinitro-1,4-dihydroquinoxaline-2,3-dione is a potent antagonist of the strychnine insensitive glycine receptor and has anticonvulsant properties. However, Sheardown et al. also disclose that 5,7-dinitro-1,4-dihydroquinoxaline-2,3-dione as well as DNQX and CNQX have poor access to the central nervous system.
International Application Publication No. WO91/13878 discloses the following N-substituted 1,4-dihydroquinoxaline-2,3-diones, which bind to the glycine receptor: ##STR8## wherein R represents hydrogen, C.sub.1-6 alkyl, or aralkyl, and n is an integer from 0 to 5; R.sup.4 represents hydrogen or hydroxy; R.sup.5, R.sup.6, R.sup.7, and R.sup.8 independently represent hydrogen, nitro, halogen, alkoxy, aryloxy, aralkoxy, C.sub.1-6 -alkyl, or aryl; R.sup.9 represents hydrogen, lower alkyl, or aryl; R.sup.10 represents hydrogen, or alkyl, and pharmaceutically acceptable salts thereof.
Leeson et al., J. Med. Chem. 34:1243-1252 (1991), disclose a number of derivatives of the nonselective excitatory amino acid antagonist kynurenic acid. Also disclosed are a number of structurally related quinoxaline-2,3-diones that are also glycine/NMDA antagonists, but are not selective and are far less potent than the kynurenic acid derivatives. The quinoxaline-2,3-diones have the structure: ##STR9## wherein R is H, 5-Cl, 7-Cl, 5,7-Cl.sub.2, 6,7-Cl.sub.2, 6,7-(CH.sub.3).sub.2, 6-NO.sub.2, or 6,7-(NO.sub.2).sub.2. Also disclosed are a number of N-methyl derivatives.
Epperson et al., Bioorganic & Medicinal Chemistry Letters, 3(12):2801-2804 (1993) report the synthesis and amino acid pharmacology of twelve N-substituted quinoxalinediones. In particular, compounds of the structure ##STR10## are reported to have significant antagonism at both the AMPA and glycine-site NMDA receptors. The functional antagonism of 4a has been demonstrated. By way of background, the authors teach that quinoxalinediones such as 6,7-dinitroquinoxaline-2,3-dione and 6-cyano-7-nitroquinoxaline-2,3-dione and 6-cyano-7-dnitroquinoxaline-2,3-dione have been shown to be AMPA (Honore et al., Science 241:701 (1988)) as well as glycine antagonists (Birch et al., Eur. J. Pharmacol. 156:177 (1988)), and also to be neuroprotective in vitro (Frandson, et al., J. Neurochem. 53:297 (1989)) and the AMPA selective quinoxalinedione 2,3-dihydroxy-6-nitro-7-sufamoyl-benzo (F) quinoxaline has been shown to be neuroprotective in cerebral ischemia models (Sheardown et al., Science 247:571 (1990)).
For a recent review on glycine antagonists, reference is made to Leeson, P. D., "Glycine-Site N-Methyl-D-Aspartate Receptor Antagonists," Chapter 13 in Drug Design for Neuroscience, Kozikowski, A. P. (ed.), Raven Press, New York, pp. 338-381 (1993).
A need continues to exist for potent and selective glycine/NMDA antagonists that: